Aluminum is a soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. Aluminium is nonmagnetic and nonsparking Aluminum powder is highly explosive when introduced to water and is used as rocket fuel. It is also insoluble in alcohol, though it can be soluble in water in certain forms. Aluminium has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded. Corrosion resistance can be excellent due to a thin surface layer of aluminum oxide that forms when the metal is exposed to air, effectively preventing further oxidation. Aluminum-carbon composites are long known to suffer from corrosion due to galvanic reaction between the dissimilar materials.